1. Field of the Invention
The present invention relates to methods for minimizing defects when transferring a useful layer of semiconductor material from a first wafer to a second wafer to produce a final structure. The final structure may be a semiconductor-on-insulator (SOI) structure that is useful for microelectronics, optics or optoelectronics applications.
2. Background Art
A typical layer transfer process includes forming a zone of weakness in a first wafer at a depth that is close to or greater than the thickness of a transfer layer. The zone of weakness is created by implanting atomic species, or by forming a porous layer, or by using any other method to weaken the mechanical bonds at a predetermined depth in the first wafer. The layer transfer process also includes bringing the surfaces of the two wafers into contact so that a surface of the transfer layer is in contact with the second wafer, and then supplying energy to detach the transfer layer from the first wafer at the zone of weakness. An additional heat treatment, known as a “stabilizing anneal”, can then be conducted to improve bonding between the second wafer and the transferred layer.
A problem associated with such a layer transfer process is that organic or inorganic contaminants or gas pockets may become trapped at the interface of the two wafers during bonding, which can detrimentally affect the efficiency and quality of the bond and thus the quality of the desired final structure. This problem is of particular concern when one or more additional treatments is/are carried out close to the bonding interface in order to modify the mechanical, physical, or chemical properties of the two wafers. In particular, such problems may occur when atomic species implantation is conducted close to a wafer surface prior to bonding to form the zone of weakness (for example, when using a “SMART-CUT®” method). Under these circumstances, contaminants or gas pockets may cause superficial blisters on the implanted wafer, and/or may lead to zones or portions of the wafer which are not transferred when the transfer layer is detached. Such defects close to the surface of the wafer are usually not reduced in number during a stabilizing heat treatment.
When conducting a SMART-CUT® process, the defects can detrimentally affect the structural properties of the detached transfer layer. In addition, such defects could cause detachment to occur at the bonding interface or defect location instead of at the implanted zone, thus creating “non-transferred” zones resulting in structural defects in the detached layer.
When such defects are detected after contact, a conventional solution for removing at least some of the defects is to break the bond between the two wafers by applying suction to a back face, then to clean the wafers and bond them together again. For example, a wafer may be scrubbed clean by using a wafer cleaner known as a scrubber. Although this process is somewhat effective, it requires breaking the bond between two wafers and then using a mechanical or chemical action on the surfaces. These processes subject the wafers to the conventional risks associated with such manipulations.